Getting the 2001 ZR 600 A Running Again
The 2001 Arctic Cat ZR 600 A is a mid-size 600cc two-stroke snowmobile that was built for trail riding. It has a belt drive, a continuous variable transmission, and a lightweight chassis that made it one of the more popular models Arctic Cat produced in that era. The engine uses a twin-cylinder layout with reed valve induction and a carbureted fuel system — specifically Mikuni VM series carbs, one per cylinder. I bought a used one about eight years ago with 4,200 miles on the clock. The seller said it ran great. It didn't. Here is what you need to know if you are dealing with one of these now.
The 2001 Arctic Cat Zr 600 A Retro Snowmobile Classic
The ZR 600 A came from the factory with a 580cc displacement (technically 584cc bore and stroke, but marketed as a 600). That was the era when Arctic Cat was stretching numbers for marketing purposes. The actual piston displacement is slightly under 600cc, but it doesn't matter much because the real story is how well those small-bore two-strokes were tuned by the factory. The primary CVT issue on these is the rear clutch sticking. After sitting even a single winter, the rear clutch halves tend to seize together from heat cycling and oil varnish buildup. I learned this the hard way. My first ride back after storage, I gunned the throttle out of a resting position and heard a loud clunk followed by a burning rubber smell. The belt was spinning in the rear clutch because the sheaves had fused shut. The fix is straightforward but requires patience. Remove the rear clutch assembly, spray penetrating oil between the sheave faces, and let it sit for at least an hour. If that doesn't free it, you need to carefully heat the outer sheave with a heat gun — not an open flame — to expand the aluminum slightly and break the seal. I once used a breaker bar against the clutch housing nut while applying heat, and that did the trick without damaging anything. Reassemble with fresh clutch grease on the splines and fresh clutch springs if they look compressed or flattened.
Another thing nobody warns you about is the intake manifold O-ring failure. The rubber manifolds that connect the carbs to the engine heads become brittle after four or five heating cycles. When they crack, you get a lean condition that manifests as hesitation on hard acceleration and difficulty maintaining idle. You will think it is a carb sync problem. It is not. The fix is replacing the intake manifold boots — parts are available from Arctic Cat and from aftermarket suppliers like Polaris parts dealers who carry the same boot dimensions. For carb sync, these Mikuni VM28 carbs need to be set to approximately 1.25 inches of intake valve lash and matched to about 1.45 inches of carb throat vacuum at idle using a proper vacuum gauge set. Without a gauge, you are guessing, and guessing gets you a rough-running machine that stalls out at trailhead parking lots. The belt itself is a 2x106 or 2x112 depending on the year and track length. The factory spec calls for a Dayco 2100 or an equivalent. Running a heavier belt like a Sterling Gold Belt will feel snappier off the line but will wear out your rear clutch faster. I ran a light performance belt for a season and replaced the rear clutch sheaves twice in twelve months. Went back to OEM spec and haven't touched the clutch since.
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One counter-intuitive thing about these engines: they are more tolerant of slightly rich carb tuning than most riders realize. Arctic Cat tuned these lean from the factory to meet emissions standards of the time. A small jet upgrade or a slight turn of the needle valve can make a bigger difference in rideability than people expect. I ran my needles two clips rich from stock and the idle settled down immediately without any loss of top end power. The exhaust did smoke more, but that is expected with extra fuel burning. Transmission oil in the secondary clutch hub is another overlooked item. Change it every two seasons at minimum. Old oil turns to sludge and causes the clutch to drag, which accelerates belt wear and reduces top speed. Drain it warm so it flows out cleanly. Use Arctic Cat OEM transmission fluid or a synthetic SAE 30. Anything heavier will cause the clutch to overheat on long climbs. Bottom line: the 2001 ZR 600 A is a solid machine if you accept that it is a twenty-plus-year-old two-stroke snowmobile. It will need attention. Not everything is wrong with it, but ignoring maintenance will compound quickly. The biggest single point of failure is the rear CVT clutch, and the second biggest is the intake manifold integrity. Address those two things first and the rest of the sled tends to take care of itself.
